1. Field of the Invention
The present invention relates to air conditioning systems and more particularly relates to forced air type air conditioning systems wherein atmospheric air is introduced into the system in controlled amounts to enable the heating or cooling load on the system to be satisfied at least in part by the atmospheric air.
2. The Prior Art
The prior art has proposed air conditioning systems in which the introduction of atmospheric air into a conditioned space or building has been controlled in accordance with sensed temperature and humidity conditions of the atmospheric air. One such proposal is disclosed in U.S. Pat. No. 2,243,647.
These proposals generally governed the introduction of atmospheric air to a system in terms of the sensed atmospheric air temperatures and relative humidity levels independently of the conditions in the zone or space which was being air conditioned.
Over the years the operating requirements for air conditioning systems have changed drastically, particularly in air conditioning systems for large apartment and office buildings. This has been due to the proliferation of electric and gas operated appliances, equipment and lighting; increased numbers of people in such structures; and, the use of construction materials which have affected heating or cooling characteristics, such as buildings having extremely large glass window areas.
It is not unusual to find office buildings in which heat generated by persons, lighting, and equipment within the building, along with heat gains from sunlight entering the building through large window areas, to be sufficiently great that the air circulating in the building need not be heated until atmospheric air temperatures are well below freezing. In such buildings it may even be necessary to provide mechanically chilled air to certain zones in order to prevent those zones from becoming uncomfortably warm even when atmospheric air temperatures are quite low.
In essence, particularly in large buildings, the wide range of air conditioning requirements from zone to zone within the building, and the sometimes great numbers of individual zones being conditioned rendered the atmospheric air temperature and humidity conditions more or less irrelevant to satisfactory operation of the air conditioning systems.
This is not to say that atmospheric air conditions were disregarded entirely, but they were frequently taken into account as secondary control parameters. An example of such a control system is disclosed in U.S. Pat. No. 3,788,386 issued to Merlin E. Demaray, in which the admission of atmospheric air to a multiple zone system was variably controlled according to sensed conditions within the air conditioned building itself. This type of air conditioning control system has been extremely successful in controlling multiple zone systems and provided for substantial usage of atmospheric air to satisfy zone load conditions.
In some circumstances however, the prior art air conditioning systems chilled the circulating air by mechanical refrigeration units, and minimized the introduction of atmospheric air into the system even though atmospheric air temperatures were relatively low, in order to adequately cool and/or dehumidify air in certain zones in which substantial amounts of heat were generated. While the efficiency of mechanical refrigeration units is relatively high when atmospheric temperatures are low, the operation of such units still requires the expenditure of energy, and to the extent this operation of the refrigeration units is avoidable the energy used in operating the units is wasted.
Generally speaking, atmospheric air can be used to heat or cool zones where the atmospheric air temperatures are, respectively, greater or less than the set point temperature of the zones. However, when the moisture content of the atmospheric air is high, or when the zones contain people and/or equipment which produce moisture in the zone air, the atmospheric air may be too moist to be suitable for heating or cooling the zone even though the temperature of the atmospheric air is at a desirable level for that purpose.
Prior art systems have been provided in which the admission of atmospheric air was governed according to sensed temperature and relative humidity of the atmospheric air but these systems have generally not been completely satisfactory because they permitted the admission of atmospheric air having moisture levels which were excessively high and also prevented the admission of atmospheric air at temperature and moisture levels suitable for conditioning the space. Such systems thus permitted admission of air which could not adequately handle the load on the space in some circumstances while in other circumstances they were inefficient because refrigerating equipment or heating equipment was operated unnecessarily.
The prior art has proposed numerous devices for sensing the relative humidity of air and controlling the operation of equipment in response to the sensed humidity. Generally these devices have controlled air humidifying or dehumidifying equipment. Examples of relative humidity sensing devices of the character referred to can be found in U.S. Pat. No. 3,163,729; 3,288,961; 3,270,153; 3,143,610; 3,115,557 and, 3,454,732.
The last mentioned patent provides for controlling humidification of air in a zone in relation to atmospheric air temperatures and the operation of the relative humidity sensing components of the device is altered according to the sensed atmospheric air temperatures. The devices disclosed by the prior art were generally responsive to sensed relative humidity and were not employed as a co-ordinated part of air conditioning control systems.